| Thanks Mike,
It appears to be the amp circuit ground. I could be confusing the amp and audio ground though.
The Ground switch (left most) seems to open the neutral wire running from cable into the main xfrmer, but seeing as I'm looking at it through a picture, I'd need to verify it up close.
I'll check up on NEC 250. Could it be that those specs weren't adhered to in the 50's?
Perhaps the switch is wired parallel with the circuit, which would explain why switching it to off or open would cancel ground circulations between the switch and the circuit. The only problem is that I don't know where the currents would then go, providing they exist.
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"Truth exists objectively even if no one apprehends it accurately." -In Search Of Certainty
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